The present invention relates to a sintered pin heater which is made of a ceramic composite structure and which has an essentially enclosed insulating layer and an external conducting layer.
German Published Patent Application No. 35 12 483 describes a ceramic heating element made of Si.sub.3 N.sub.4 /MoSi.sub.2 composites having a proportion of 35-75 mole percent of Si.sub.3 N.sub.4, the average particle diameter of the used Si.sub.3 N.sub.4 powder being twice as large as that of the used MoSi.sub.2 powder. The average particle diameter of the MoSi.sub.2 powder is 2 .mu.m or smaller.
However, the utilization of this powder combination leads to satisfactory strengths only if an axial hot-pressing or a hot isostatic pressing process is used. However, this method has the disadvantage that a hard machining requiring considerable outlay must be carried out subsequently.
German Published Patent Application No. 35 19 437 describes an electrical, ceramic heating device, likewise using Si.sub.3 N.sub.4 /MoSi.sub.2 powders, the electrically insulating part being made of powders whose average particle diameter is 1-50 .mu.m. The conductive powder has the same or a larger average particle diameter than the insulating powder. The conductive part of the heating device is designed in such a manner that the electrically conductive powder is not larger than half the average size of the electrically insulating powder. In this case, as well as in German Published Patent Application No. 35 12 483, this powder combination leads to products having a sufficient strength only if an axial hot-pressing or a hot isostatic pressing process is used with the above-mentioned disadvantages.
German Patent No. 37 34 274 describes ceramic composites on the basis of silicon nitride, aluminum nitride, and .beta. sialon in combination with secondary phases from different silicides, carbides, borides, and nitrides of transition-metal elements. Depending on the secondary phase content, these materials possess selectively adjustable electrical properties. The adjustable specific values for the electrical resistance of these materials at room temperature lie between 1.multidot.10.sup.13 to 1.multidot.10.sup.-4 .OMEGA.cm and exhibit a positive dependence on the temperature (PTC effect). The strength level of these composites produced in this manner does not lie below 200 MPa. The method used there for manufacturing highly heat-resistant composites is to be considered a uniaxial hot-pressing which, in particular, has disadvantages with respect to the shaping of bodies manufactured from these composites, as mentioned above. Further disadvantages are that bodies made therewith can have anisotropic material properties because of the pressing direction and that the method is only usable as batch process, i.e., not as continuous process. Moreover, this method requires high temperatures and pressures.
Also described in German Patent No. 37 34 274 is the implementation of a ceramic heater or a sheathed-element glow plug using Si.sub.3 N.sub.4 /MoSi.sub.2 composites having sintered-in metal wires as supply leads.